1. Field of the Invention
The present invention relates to a process for preparing yellow iron oxide pigments by the precipitation method from iron(II) chloride or iron(II) sulphate raw material and calcium carbonate precipitant.
2. Brief Description of the Prior Art
The precipitation method of preparing yellow iron oxide pigments has been known for a long time. The practical implementation of this method is described for example in Ullmann's Encyclopedia of Industrial Chemistry, 5th Ed., Vol A20, p. 297 ff. The raw material used is usually iron(II) sulphate obtained in the pickling of sheet steel or in the production of titanium dioxide by the sulphate process.
Within the pickling industry, however, an increased trend has been observed over a number of years towards the use of a hydrochloric acid pickling agent. The resultant iron(II) chloride has a purity which makes it especially suitable for preparing yellow iron oxide. Additionally the chloride process is being employed to an increasing extent worldwide for producing titanium dioxide. From both sources, therefore, there is an increased supply of solutions containing iron(II) chloride, which as far as possible should be converted to a substance of value. One common process for doing this is the spray roasting process, in which the iron(II) chloride or iron(III) chloride is subjected to oxidative hydrolysis at high temperatures (typically more than 1000° C.). End products produced here are iron oxide, typically hematite, and hydrochloric acid, which as a desired substance of value is introduced back into the pickling operation. Without further purification steps the iron oxide obtained is generally suitable for producing hard ferrites (permanent magnets).
Where readily magnetizable soft ferrites are to be produced from such iron oxides, the iron chloride solution must be subjected to additional purification operations, which make the process much more costly. Since increasing quantities of iron chloride, largely of low quality, are being made available and since the capacity of the ferrite market to absorb this supply is limited, an alternative is sought for producing a substance of value from these iron chlorides. Direct land filling or marine dumping of the iron chloride solutions is not possible, on environmental grounds.
In general α-FeOOH (yellow iron oxide) is prepared from iron(II) salts by the precipitation method, as described in DE-A 2 455 158, for example, or by the Penniman process, as described in U.S. Pat. No. 1,368,748 and U.S. Pat. No. 1,327,061. In both processes a nucleus is first prepared onto which then, in a further—relatively slow step—further α-FeOOH is caused to grow. Generally speaking, FeCl2, unlike FeSO4, is not isolated as a crystalline substance, since it has a much higher solubility in water as does FeSO4. It is therefore contaminated to an extent many times greater than FeSO4, a fact which can prove to be very disadvantageous for pigment production.
It is known that many iron(II) chloride solutions from steel pickling do not lead to pure acicular α-FeOOH nuclei when the so-called acidic nucleus process is employed; on this point see EP-A-0 406 633, Example 1, where the only product preparable was a brown powder. In many cases, instead of α-FeOOH, β-FeOOH is obtained in the form of very long, thin needles. In the course of the subsequent formation of pigment, depending on the temperature used, β-FeOOH undergoes partial breakdown into α-Fe2O3 (Chambaere, D. G. & De Grave, E.; Phys. Chem. Minerals; 12, (1985), 176–184). It is not possible to use β-FeOOH as nucleus material for preparing α-FeOOH pigments. In order nevertheless to be able to use the inexpensively available FeCl2 from steel pickling for preparing α-FeOOH nucleus, it is preferred to prepare the nucleus by what is called the alkaline process, as described in U.S. Pat. No. 2,558,304, for example. Very often, however, such nuclei lead to colour values which are found to be somewhat light. On this point see, for example, EP-A 0 911 370 (L* values of from 62.0 to 64.0). For precipitation in the case of pigment synthesis it is general practice to use an alkaline precipitant. Normally NaOH, Na2CO3 or ammonia is used for this purpose (EP-A 911 370). Surprisingly it has been found that CaCO3 and/or dolomite can also be used as precipitant provided that it is used in finely ground form (particle size <90 μm) as an aqueous suspension or as powder.
Industrial iron(II) chloride solutions further frequently include organic constituents (e.g. pickling assistants) and relatively large quantities of higher polyvalent cations such as Ti, Cr, Al, V and Si, for example, which can have a disruptive effect on pigment growth. The cations can be separated off by precipitation with an alkaline component. On this point see, for example, EP-A 0 911 370. The iron chloride solution resulting from such an operation has a pH of from 2 to 4.
It was an object of the present invention, therefore, to prepare by the precipitation method a yellow iron oxide pigment which has relatively dark L* values and uses preferably iron(II) chloride raw material.